*Article* **Optimization of Oleuropein and Luteolin-7-O-Glucoside Extraction from Olive Leaves by Ultrasound-Assisted Technology**

### **Antonio Lama-Muñoz \*, María del Mar Contreras, Francisco Espínola, Manuel Moya, Inmaculada Romero and Eulogio Castro**

Department of Chemical, Environmental and Materials Engineering, University of Jaén, Campus Las Lagunillas, s/n, Building B3, 23071 Jaén, Spain

**\*** Correspondence: alama@ujaen.es; Tel.: +34-953-211-860

Received: 29 May 2019; Accepted: 27 June 2019; Published: 28 June 2019

**Abstract:** The olive orchard cultivation in Mediterranean countries results in huge amounts of lignocellulosic biomass residues. One of the main residues are olive leaves. Olive leaves contain high concentrations of bioactive antioxidant compounds like oleuropein and luteolin-7-glucoside. The production of biactive compounds from olive leaves requires treatments capable of breaking the lignocellulosic structure. Current research focuses on use of inexpensive, quick, and not harmful to the environment treatments, searching a more simplified large-scale operation approach. Recently, advances in applied chemistry have led to possible new emerging industrial techniques like ultrasound-assisted extraction (UAE). This technology is a promising candidate as a green treatment solution for olive leaves utilization in a biorefinery. However, this application goes through prior optimization of technique and operating conditions. The goal of this study was to optimize the extraction of oleuropein and luteolin-7-glucoside from olive leaves through an investigation of the influence of key factors of ultrasound-assisted extraction using an experimental central composite design, in comparison with conventional Soxhlet extraction. The highest extraction efficiency and antioxidant capacity were obtained under optimal increment of temperature and amplitude conditions (40 ◦C and 30%, respectively). Values for oleuropein, luteolin-7-glucoside were 69.91 g/kg and 1.82 g/kg, respectively.

**Keywords:** antioxidants; bioactive compounds; biorefinery; experimental design; olive leaves; olive biomass; response surface methodology; ultrasound-assisted extraction; olive byproducts valorization
